In recent years, the SOC (State of Charge; a charging state, and remaining capacity) of a battery has been calculated on the integrated value of a charging/discharging current of the battery. However, when SOC is calculated only by the integration of the charging/discharging current of the battery, a current detection error is accumulated in the integration and the state of charge, which is the calculation result after a predetermined period of time has elapsed, it is likely to be greatly different from the actual value. In order to avoid this phenomenon, Patent Document 1 discloses a technique which estimates a battery open voltage (the voltage of the battery from which the internal impedance of the battery is removed) when the charging/discharging of the current are inverted, calculates the state of charge from the battery open voltage, and corrects the state of charge calculated from the integration of the current. In addition, Patent Document 2 discloses a technique which estimates a battery impedance voltage, subtracts the battery impedance voltage from the measured voltage to calculate a battery open voltage, estimates the state of charge from the battery open voltage, and corrects the state of charge from the relationship between the battery open voltage and the state of charge calculated from the current.